The present study is aimed at the understanding of the molecular and cellular mechanisms that regulate the transcriptional activity of the myogenic bHLH proteins. More specifically, we are interested in studying the switches that allow MyoD to become transcriptionally active once undifferentiated myoblasts transition into differentiated myotubes. MyoD activity is required for proliferation of satellite cells and is deregulated in rhabdomyosarcoma, a pediatric solid tumor. The molecular mechanisms of regeneration and neoplastic transformation of skeltal muscle cells remian unclear. Acetylation is a dynamic process involving the actions of both acetyltransferases and deacetylases. MyoD is a direct target of acetylation by both p300 and PCAF. Nonetheless, acetylation mediated by PCAF, but not by p300, is required to assist MyoD-dependent transcription and muscle differentiation. PCAF-mediated acetylation of MyoD results in an increased DNA binding capability and modify the conformation of MyoD bound to DNA. We have previously analyzed the contribution of the p300 and PCAF acetyltransferases to muscle differentiation. We propose to continue this line of investigation. The specific aims are:1. To investigate whether the HDACs physically associate with MyoD and impede muscle differentiation. 2. To analyze the functional interplay of MyoD, pRb and HDAC1 in regulating muscle specific transcription.3. To investigate the role of acetylation in both regeneration and neoplastic transformation of skeletal muscle cells. In the past year, we have begun characterizing the composition of a cellular complex containing MyoD and HDAC1 and have initiated experiments of HDAC1 overexpression in muscle cells. Our preliminary results indicate that forced expression of HDAC1 antagonizes MyoD-dependent transcription and prohibits muscle differentiation.